receiver/prometheus: migrate code in here

[odeke-em]

Hand over prometheus exporter code that I had
parked in https://github.com/orijtech/promreceiver

That code was written for OpenCensus Authors, and was just
waiting for a final destination. This change finishes that.

Fixes #480

[codecov]

Codecov Report

Merging #481 into master will increase coverage by 0.46%.
The diff coverage is 82.31%.

@@            Coverage Diff             @@
##           master     #481      +/-   ##
==========================================
+ Coverage   57.02%   57.49%   +0.46%     
==========================================
  Files          69       68       -1     
  Lines        4247     4759     +512     
==========================================
+ Hits         2422     2736     +314     
- Misses       1666     1844     +178     
- Partials      159      179      +20

Impacted Files	Coverage Δ
receiver/prometheusreceiver/metrics_receiver.go	73.46% <100%> (ø)	⬆️
receiver/prometheusreceiver/buffering.go	100% <100%> (ø)
receiver/prometheusreceiver/options.go	66.66% <66.66%> (ø)
receiver/prometheusreceiver/receiver.go	80.15% <80.15%> (ø)
...servability/observabilitytest/observabilitytest.go	0% <0%> (-100%)	⬇️
exporter/opencensusexporter/opencensus.go	48.21% <0%> (-3.64%)	⬇️
...al/collector/processor/nodebatcher/node_batcher.go	77.72% <0%> (-0.46%)	⬇️
cmd/occollector/app/collector/processors.go	0% <0%> (ø)	⬆️
exporter/exportertest/nop_exporter.go	100% <0%> (ø)	⬆️
exporter/loggingexporter/logging_exporter.go	100% <0%> (ø)	⬆️
... and 7 more

---

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[songy23]

Suggestion: this PR is too huge to review, consider putting the parsing logic in a util file and have a separate PR for it first.

[pjanotti]

Hi @odeke-em, I'm closing this PR since it is stale for some time and iirc was superseded by other (later closed), let's know if that is not correct.